If you want to use a meter with tubes, use 6AL5s for signal rectifying diodes (or 1N4148 if you don't care) and do it passively, or add, at worst, a preamp/driver stage consisting of a gain stage with cathode follower hanging off it. Or you can put the diode(s) before an amplifier, though you have to pay more attention to DC stability. You might want to read up on VTVM (Vacuum Tube VoltMeter) schematics, too.